A variety of coating compositions having a liquid medium, generally at least substantially resin-free, which can be chromium-containing coating compositions, but that can also include those that don't contain chromium, and all typically for protecting ferrous substrates, are known. Of special interest are those which contain particulate metal. Representative coating compositions of this type that were initially developed could be quite simplistic, such as compositions containing essentially chromic acid and particulate metal in an alcohol medium, as disclosed in the U.S. Pat. No. 3,687,738.
A later development of particular effectiveness for providing a corrosion-resistant coating on metal substrates was the more complex composition such as shown in U.S. Pat. No. 3,907,608. The composition comprised chromic acid, or equivalent, a particulate metal of mainly zinc or aluminum, wetter and a liquid medium comprising water plus high-boiling organic liquid. The composition had very desirable coating characteristics when including a viscosity modifier such as a water-soluble cellulose ether, as disclosed in U.S. Pat. No. 3,940,280.
The coating could be especially useful as an undercoating. Thus it has been taught to use such a more complex coating composition as an undercoating over ferrous surfaces. The coating is then provided with a silicate topcoating, as disclosed in U.S. Pat. No. 4,365,003.
It has been known that where coating compositions could contain the particulate metal as untreated aluminum flake, such flake can be unstable in water-based coating compositions. In such water-based coating compositions, standard aluminum flake will react with water in the composition to form hydrogen gas. One approach for avoiding this problem has been to coat the aluminum flake. One such coating is an acrylic coating formed by reacting mono-ethylenically unsaturated silane with acrylic monomers having amine hydroxyl or epoxy groups, as disclosed in U.S. Pat. No. 4,213,886. However, these products are specialty items tailored to provide a coating of good glamour appearance and have not found a wide acceptance.
There has also been proposed the preparation of coating compositions that contain hydrolyzed organotrihydrocarbonoxy silane and a particulate metal. These compositions, such as disclosed in U.S. Pat. No. 4,218,354, can provide corrosion protection to a coated substrate. The silanes utilized are not water-reducible and, thus, it can be expected that the compositions are best formulated in the presence of organic liquid.
More recently, it has been taught in U.S. Pat. No. 5,868,819 that composition substituents which are epoxy functional silanes, and which are water-reducible, can be useful in forming compositions for coating metal substrates. The compositions rely on a variety of ingredients to provide for a chrome-free system.
Other compositions containing particulate metal and finding utility by providing corrosion protection for a substrate are well known. Some of these will be more particularly discussed hereinbelow. It would be desirable to provide a coating from all such compositions, and also to provide a coating combination of undercoating plus topcoating, each of which could have wide acceptance. It would further be desirable to provide same, which would offer outstanding corrosion protection to metal substrates and be efficiently and economically produced.